1. Field of the Invention
The present invention relates to a method for forming an insulating film on a semiconductor substrate surface. More particularly, the present invention relates to a method for forming a silicon nitride film, which exhibits an excellent stability in electrical properties and a high degree of density, on a surface of a semiconductor substrate, which film covered semiconductor substrate is useful for the production of integrated circuits (IC).
2. Description of the Prior Art
In the production of semiconductor devices, it is known to coat a portion of the semiconductor surface, in which a PN junction is exposed to an ambient atmosphere, with an insulating film in order to prevent changes in the electrical properties of the semiconductor device with the lapse of time and to enhance the reliability of the semiconductor device. In the production of MISFET (metal insulator semiconductor field effect transistor) or MIS type integrated circuits, it is also known to form a gate insulating film in the semiconductor device, so as to introduce an inversion layer channel into the device. Generally speaking, in the conventional MISFET and MIS type integrated circuits, the semiconductor substrate, the insulating film and the metal gate electrode, respectively, consist essentially of silicon (Si), silicon oxide (SiO.sub.2) and aluminum (Al).
The above-mentioned structure of the conventional metal insulator semiconductor device, however, exhibits a disadvantage in that an application of an electric field of about 10.sup.6 V/cm across the insulating film at a temperature of about 200.degree. C. results in a remarkable fluctuation of a gate threshold voltage. This disadvantageous phenomenon may be due to the fact that the drift effect of impurity ions contained in the SiO.sub.2 insulating film or the structural defects of the SiO.sub.2 insulating film per se cause the creation of a capture level of carriers, and the creation of the capture level of carriers results in a remarkable fluctuation in the space charge distribution in the surface layer of the semiconductor device. It is clear that the fluctuation in the space charge distribution is influenced most remarkably by the structural defects in the interface between the Si substrate and the insulating film (SiO.sub.2). The structural defects are created when the insulating film (SiO.sub.2) is formed by thermally oxidizing the surface layer of the Si substrate. This thermal oxidization process tends to permit the impurity ions, such as alkali metal ions, to contaminate the oxidized silicon film.
In order to eliminate the above-mentioned defects from the conventional insulating film, attempts have been made to provide an insulating film by way of chemical vapor deposition or sputtering. However, all such attempts have failed to reduce the structural defects in the interface between the Si substrate and the SiO.sub.2 layer to a level lower than that resulting from thermal oxidation.
Furthermore, it is known that when the surface of the Si substrate is thermally oxidized in an extremely clean atmosphere, the resultant SiO.sub.2 film exhibits very little structural defects at a level of surface charge density of 10.sup.11 /cm.sup.2 or less. However, the interface between the Si substrate and the SiO.sub.2 film still exhibits structural defects due to excessive silicon ions. Accordingly, it has been strongly desired to eliminate the above-mentioned defects from the interface between the Si substrate and the SiO.sub.2 film.
It is known to form a silicon nitride film by a so called plasma CVD process as illustrated in FIG. 1. Referring to FIG. 1, a nitrogen containing gas, such as nitrogen (N.sub.2) or ammonia (NH.sub.3), is admitted through a first inlet 10 into a reaction chamber 11, while a silicon containing gas, such as monosilane (SiH.sub.4), is admitted through a second inlet 12 into the reaction chamber 11. The reaction chamber 11 is evacuated by a not shown vacuum pump connected to an outlet 13. A radio frequency current of, for example, 13.65 MHz, is applied by means of a coil 14 to the nitrogen containing gas in the reaction chamber 11, so as to form a gas plasma. A silicon wafer 15 is heated by a heater 16 to a temperature of from 100.degree. to 500.degree. C. and a silicon nitride is deposited on the silicon wafer due to a reaction between the monosilane and nitrogen in the plasma state. Instead of induction coupling by the radio frequency current, the gas plasma can be created by a capacitance coupling between a pair of electrodes. The silicon nitride film produced by the plasma CVD process exhibits a relatively low stoichiometry and, therefore, not stoichiometric ions, such as silicon ions, cause generation of defects at the interface between the silicon nitride film and the silicon wafer.
It is already known that a nitridation reaction of silicon proceeds by heating a silicon substrate to an elevated temperature in a nitrogen or ammonia atmosphere. However, the nitride film produced by such a nitridation reaction disadvantageously includes a high concentration of oxygen resulting from ammonia and a nitridation reaction tube made of quartz. In addition, it is difficult to form a uniformly amorphous film of silicon nitride. As a result, the silicon nitride film produced by the known nitridation reaction disadvantageously exhibits a low resistivity due to current conduction through the grain boundaries of a locally polycrystalline film of silicon nitride. Furthermore, in the known nitridation reaction, the nitridation reaction requires a high temperature and long reaction time for forming a nitride film having a thickness more than 50 A. For example, in the case of using ammonia gas, a nitridation reaction temperature of 1200.degree. C. and a nitridation reaction time of 200 minutes or longer are required for forming a nitride film of such thickness.
If the thickness of nitride film is 50 A or less, the resistivity thereof is too low to use the film as an insulating film due to a tunnel current through the nitride film.
It is known from European Laid Open Application No. 0006706 (European Patent Application No. 7930114.9, filed by Fujitsu Limited) to form an insulating film by a direct nitridation in a nitrogen or ammonia atmosphere of a silicon dioxide film which is formed on a silicon substrate. A dense structure of the insulating film is provided by the nitridation of the silicon dioxide film. However, it was found by the present inventors that a relatively high concentration of oxygen in the insulating film causes generation of structural defects at the interface between the insulating film and the silicon substrate.